Block Queue solves one of the major shortcomings of Drupal 6 block management: the ability to easily re-use blocks in different regions, on different pages, with arbitrary ordering.
Block Queue allows users to create "queues" of blocks much like Nodequeue allows to create queues of nodes: by adding arbitrary blocks to the queue and ordering them any way necessary. Queue size can be limited.
Queues are exposed as blocks and can be easily included in Drupal regions.